Thermoplasmonic-induced energy-efficient catalytic oxidation of glycerol over gold supported catalysts using visible light at ambient temperature
Graphical abstract
Introduction
Gold Nanoparticles (Au-NPs) have been found to be highly effective nanocatalysts in many reactions for chemical and energy transformations [1,2]. In addition to their high catalytic activity, Au-NPs exhibit unique optical and electrical properties thanks to the localized surface plasmon resonance (LSPR) effect [3,4]. The excitation of LSPR of Au-NPs under visible light gives many valuable physical effects such as optical near field enhancement, generation of heat and energetic electrons [5]. These physical effects are very promising in the specific context of light-induced chemical reactions. Indeed, the recent investigations of LSPR in chemistry have led to novel approaches, where the reaction is initiated, either activated or strongly promoted, in the presence of photo-excited Au-NPs. One can mention here the plasmon-induced near-field photochemistry at the nanoscale [6,7], and the plasmon-assisted photocatalysis, where hot energetic electrons generated at the surface of photoexcited Au-NPs can be transferred to an adjacent acceptor such as molecules or semi-conductors in order to activate some photodegradation [[8], [9], [10], [11], [12]] or hydrogen production [[13], [14], [15]] reactions. In this context, recent investigations have shown that Au-NPs can act as a local source of heat to activate some reactions at the nanoscale. This promising feature of Au-NPs is advantageously coupled to their catalytic properties in order to drive heterogeneous catalytic reactions at room temperature without external heat sources [[16], [17], [18], [19], [20], [21], [22]]. This approach was proposed first for the production of 4-benzoylmorpholine from benzaldehyde and morpholine in the presence of an Au/SiO2 catalyst under laser irradiation (532 nm) at room temperature [19]. Then, some reactions catalyzed by Au-NPs under visible irradiation have been studied, such as oxidation of 9-anthraldehyde [23] and Ethylene Glycol [24], ketones reduction to alcohols or deoxygenate epoxides reduction to alkenes [25]. The ability of Au-NPs to act simultaneously as a catalyst and a local heat source under visible irradiation thus opens the way to investigate such an approach and to assess the driving parameters towards more valuable industrial reactions.
For example, the use of glycerol (GLY), the main coproduct in biodiesel production [[26], [27], [28]], has been widely studied, and a lot of investigations have been focused on its catalytic upgrading to high value added products for, e.g., industrial and pharmaceutical applications [[29], [30], [31], [32], [33]]. Many pathways are mentioned for the transformation of GLY [[34], [35], [36]], particularly in the liquid phase [[37], [38], [39], [40], [41]]. More particularly, the use of gold-based supported catalysts has been extensively investigated. Over this type of catalysts, the reaction should be carried out in basic medium, at moderate temperatures (60–90 °C), under an oxygen pressure of up to 10 bars [29,42].
Recently, conventional thermally driven catalytic oxidation at 90 °C of GLY was combined to visible light irradiation in order to enhance conversion in the presence of Au-NPs [43,44]. The results confirmed the beneficial effect of visible light irradiation compared to the reaction performed in the dark and revealed the contribution of plasmonic effect to enhance the oxidation reaction of GLY. However, the maximum value of conversion, reached under heating and illumination, was 55% after 5 h of reaction, which was actually slightly higher than the 35% conversion obtained in the dark conditions. Additionally, to the best of our knowledge, no catalytic glycerol conversion has been performed so far exclusively under illumination-induced activation.
Herein, we demonstrate for the first time, the feasibility of efficient GLY oxidation using only laser radiation without any additional heat source over gold nanoparticles-based supported catalysts. We showed that the laser excitation of the plasmon band of Au-NPs in the presence of a base and oxygen at atmospheric pressure, resulted in the improved formation of glyceric and tartronic acids as main products. The highest observed conversion was close to 90% after 2 h of reaction, while the reaction did not occur in the absence of laser irradiation thus confirming the contribution of plasmonic heating in the oxidation of GLY. The influence of the laser power on the conversion rate was evaluated. Comparing various supports, the contribution of plasmonic photocatalysis on the oxidation reaction through the coupling between photoexcited Au-NPs and the TiO2 support was especially highlighted.
Section snippets
Materials and reagents
The 1 wt.% Au/TiO2 and 1 wt.% Au/Al2O3 commercial catalysts under the form of extrudates were purchased from STREM Chemicals. The specific surface areas (SSAs) of these catalysts are ca. 50 m2/g and 230 m2/g, respectively. Both catalyst exhibits a dark purple color with 2–3 nm Au-NPs showing a plasmon resonance band around 532 nm (Figure S1). For catalytic experiments, each commercial catalyst was crushed by mortar and pestle for 20 min.
Additional pure commercial TiO2 extrudates (anatase, 107 m2
Results and discussions
As shown in Table 1, the catalytic tests were performed with the Au/TiO2 and Au/Al2O3 catalysts. Additional blank tests without any catalyst or with only TiO2 or Al2O3 supports were also performed. The measured temperatures during catalytic tests as the function of the reaction time, obtained in each run, are summarized respectively in Fig. 2(a) and (b).
First, in blank test without any catalysts, the solution temperature was stable at room temperature (i.e., 25 °C) after 1 h of illumination
Conclusions
Herein, we show that supported Au-NPs can be effectively used for glycerol oxidation in the absence of any external heat source. By exciting exclusively plasmonic nanoparticles with a LSPR band at 532 nm, it was possible to convert up to 90% of glycerol after 2 h at atmospheric pressure; under our conditions, the laser activation led to a conversion 2.5 times higher than when using thermal activation at 60 °C. The comparison between the reactions in the presence and the absence of laser
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Acknowledgment
Financial support of the "Conseil Régional Champagne-Ardenne", (Projet Catalyse Nanoplasmonique), NanoMat (www.nanomat.eu) by the "Ministère de l’enseignement supérieur et de la recherche", the "Délégation Régionale à la Recherche et à la Technologie" of Champagne Ardenne (projet Catalyse Nanothermique) is acknowledged. Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur et de la Recherche, Région Hauts-de-France, CNRS, FEDER, Centrale Lille and Centrale Initiatives Foundation
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Present Address: Sorbonne Université, UPMC Univ Paris 06, CNRS, Collège de France, Chimie de la Matière Condensée de Paris, 75005 Paris, France.